1. Field of the Invention
The present invention relates to a light amplifier and a light amplifying method and particularly, to a light amplifier having a dummy light-inserting function and a light amplifying method therefor.
2. Description of the Related Art
A light amplifier and a light amplifying method have been hitherto applied for light amplification, for example in an optical communications system. Recently, explosive propagation of the Internet has urged a basic transmission system to rapidly increase its transmission capacity. In order to increase the transmission capacity by effectively using the present infrastructure, much attention has been paid to WDM (Wavelength Division Multiplexing), particularly DWDM (Dense Wavelength Division Multiplexing) having high density in the wavelength division multiplexing. A light amplifier and a light amplifying method have held high position as a means for collectively amplifying light signals which are subjected to the wavelength division multiplexing.
FIG. 1 shows a conventional general light amplifier.
The light amplifier comprises directional couplers 301 and 304, excitation light couplers 302 and 303, light amplifying medium 300, front excitation light source 306, rear excitation light source 307, photo detectors 305 and 308, and control circuit 360.
Light signal 401 from a transmission path is split into two parts by the directional coupler 301, and one part thereof is converted to the electrical signal corresponding to input power by the photo detector 305. The output of the light amplifying medium 300 is split into two parts by the directional coupler 304, and a part thereof is converted to the electrical signal corresponding to output power to a transmission path by the photo detector 308. Light signal 403 is outputted from the light amplifier.
The control circuit 360 controls the power of each excitation light of the front excitation light source 306 and rear excitation light source 307 so that the output power to the transmission path is equal to predetermined power. The excitation light source may be disposed at only one of the front and rear sides.
If any trouble occurs at a terminal station serving as a transmission end of light signals or the like and thus the number of channels for the light signals of the transmission path is greatly reduced, the light amplifier is controlled to keep a predetermined light output at remaining wavelength numbers.
In the conventional technique described above, if any trouble occurs at a terminal station serving as a transmission end for the light signals and thus the number of channels for the light signals of the transmission path is greatly reduced, for example, the light amplifier is controlled to keep a predetermined light output at remaining wavelength numbers. As a result, the light sources for the remaining wavelengths are forced to power up. Further, generally, the control of the output of the light amplifier cannot sufficiently follow the rapid variation of the light power, and thus the light output of the transmission is greatly varied. Therefore, the conventional technique has also a disadvantage in keeping the quality of the transmission path.
The present invention has an object to provide a light amplifier and a light amplifying method which can prevent overpower of signal light of remaining wavelengths when the number of channels of multiplexed signals is rapidly reduced due to some trouble at a terminal station or the like.
In order to attain the above object, according to a first aspect of the present invention, there is provided a light amplifier comprising: a dummy light source for emitting dummy light; a first directional coupler for combining a light signal with the dummy light; a second directional coupler for splitting the composite light of the light signal and dummy light outputted from the first directional coupler; a light amplifying portion for amplifying the one part of the composite light outputted from the second directional coupler; a dummy light removing filter for removing the dummy light from the amplified composite light which is outputted from the light amplifying portion; and a dummy light control circuit for controlling the dummy light source so that the light output of the dummy light source is equal to a predetermined light output, wherein lack of the light input power due to reduction of the number of channels for light signals of a transmission path is compensated by the dummy light.
According to a second aspect of the present invention, there is provided a light amplifying method comprising: a dummy light emitting step of emitting dummy light; a first directional coupling step of combining a light signal with the dummy light; a second directional coupling step of splitting the composite light of the light signal and dummy light combined with said first directional coupling step; a light amplifying step of amplifying the one part of the composite light outputted with said second directional coupling step; a dummy light removing step of removing the dummy light from the amplified composite light which is outputted with said light amplifying step; and a dummy light control step of controlling the dummy light source so that the light output of the dummy light is equal to predetermined light output, wherein lack of the light input power due to reduction of the number of channels for light signals of a transmission path is compensated by the dummy light.